The present invention relates to a method for operating an active chassis system.
DE 38 44 803 C1 discloses an active chassis system whose support assemblies can be used to set a targeted change in the supporting forces. When deviations occur in the rolling angles and pitching angles in relation to the respective desired values, changes in the supporting forces and associated wheel contact forces are changed actively. Furthermore, the support assemblies provide the possibility of selectively counteracting, at individual wheel suspension means, wheel movements which adversely affect the ground grip of the wheels.
DE 37 05 520 A1 discloses a control unit for influencing the wheel contact forces of a vehicle which automatically reduces a deviation of the vehicle from the course predefined by a steering wheel lock.
An object of the present invention is to permit a change in travel direction by actively changing the wheel contact forces of the vehicle in further driving situations.
This object has been achieved by a method on which a desired yaw rate is determined from the information of a device arranged in the vehicle for the purpose of determining the profile of the roadway in a control unit, and the wheel contact forces are set as a function of the desired yaw rate. The relationship between the desired yaw rate, i.e., a desired yaw rate and the wheel contact forces, is described by means of a characteristic curve or a calculation model.
The device for determining the profile of the roadway senses, for example, the profile of the road, road markings, boundary posts, obstacles or other road users. This information is used as a basis for calculating a desired yaw rate. The device for determining the profile of the road can be implemented, for example, as an optical camera for detecting the environment or as a radar device.
The desired yaw rate is preferably calculated in a control unit which actuates actuating elements which interact with the support assemblies. This control unit can also be combined with other control units, for example a control unit for an electronic stability program (ESP). By actuating the actuating elements, unequal contact forces can be set at the wheels of an axle, unequal side forces also occurring at the wheels. As a result, a resulting side force acts on the axle so that a yaw moment which acts at the center of gravity of the vehicle is produced. This method advantageously changes the direction of travel of the vehicle without the driver's intervention, during which process no disruptive reaction moments occur at the steering wheel.
In a further refinement of the invention, the desired yaw rate is calculated as a function of a velocity and/or a yaw rate. In order to determine the desired yaw rate, it is necessary not only to have information from the device for determining the profile of the roadway but also information about the current travel state which is described by means of the velocity, the yaw rate and the steering wheel angle. This information is advantageously made available to the control unit by a data bus. When the vehicle deviates from a desired course due to interference influences, the desired yaw rate which is to be set by the actuating elements depends on the velocity of the vehicle. To the same degree, the steering wheel angle is necessary to correct the direction of travel of the desired direction of travel predefined by the driver by way of a steering wheel. The information about the current travel state permits the direction of travel of the vehicle to be influenced in a reliable and convenient fashion.
In a further refinement of the invention, a desired course is calculated from the information from the device for determining the profile of the roadway. The motor vehicle is guided along the desired course by actuating the actuating elements. The device for determining the profile of the roadway senses information about the environment such as side strips or median strips, lane boundary markers such as crash guard rails or road posts. A virtual desired course is determined from the profile of the roadway and the vehicle is guided along this desired course by changing the wheel contact forces by actuating the actuating elements.
In a still further refinement of the invention, the device for determining the profile of the roadway detects a deviation of the direction of travel of the vehicle from the direction of travel predefined by the driver and is compensated by actuating the actuating elements. These deviations can have numerous causes such as side wind, lateral inclination of the roadway or track grooves. Alternatively, or in addition to the device for determining the profile of the roadway, a deviation in the direction of travel of the vehicle from the direction of travel predefined by the driver can also be detected by evaluating wheel speed differences, steering angles and/or steering torques. The deviations are advantageously compensated by changing the wheel contact forces without the driver feeling disruptive feedback at the steering wheel.
In a further refinement of the invention, an obstacle is detected by the device for determining the profile of the roadway and is avoided by actuating the actuating elements. As soon as the device for determining the profile of the roadway detects an obstacle, a desired yaw rate is calculated. The actuating elements are actuated as a function of the desired yaw rate and an obstacle is avoided. The necessary data such as distance from and dimensions of the obstacle which are necessary to calculate the desired yaw rate can be supplied, for example, by a radar device or an optical camera.
In a yet further refinement of the invention, when straight-ahead travel is detected by the device for determining the profile of the roadway and when there is steering torque which is permanently present, the actuating elements are actuated so that the steering torque is reduced while the direction of travel is maintained. With this method, certain fabrication tolerances or even slight changes in the axle geometry owing to damage by changes in the wheel contact forces can be compensated so that the steering torque is reduced for straight-ahead travel, or is at best zero.
The system which comprises the device for determining the profile of the roadway and a steering torque measuring device detects that in the case of straight-ahead travel a continuous steering torque is present. By correspondingly changing the wheel contact forces, the steering torque ideally becomes zero so that the vehicle is made to travel straight ahead without steering torque. The straight-ahead travel can alternatively also be detected by a satellite-supported locating system such as GPS (Global Positioning System).
In addition to or as a simple alternative to the device for determining the profile of the roadway, the driver can be relieved of continuous steering torque even via a steering torque measuring device and/or a steering angle sensor. For example, by an evaluation of the steering torque over a relatively long time period can detect that the driver has to continuously apply a steering torque when traveling straight ahead. By changing the wheel contact forces, the steering torque is reduced or eliminated. An additional steering angle sensor permits easier detection of the straight-ahead travel. In conjunction with a device for determining the profile of the roadway, signals of the steering angle sensor can also be used for checking the plausibility of the direction of travel.
The wheel contact forces (F11-F14) can be set by actuating elements which change the prestressing of a helical spring or of an air spring or of a hydraulic spring. The wheel contact forces can be set by varying the prestress of the spring.
In another embodiment of the invention, the wheel contact forces (F11-F14) are set by actuating elements which change the prestressing of a stabilizer. The system utilizes the already existing stabilizer and is thus cost-effective to perform.
The toe-in angle and the associated slip angle α at a front axle and/or a rear axle can be changed as required by an adjustment element. The level of the yaw moment which can be generated by changing the wheel contact forces is dependent on the toe-in angles which are set. Since a permanently set large toe-in angle causes a high level of wear of the tires and high fuel consumption, an adjustment element is provided which increases the toe-in angle only when required. This adjustment element is arranged, for example, in track rods of the front axle and/or rear axle. The level of the yaw moment which is necessary to correct the direction of travel is thus advantageously increased.
In a still further embodiment of the invention, a desired yaw rate is determined from at least one of the sensed values such as wheel speed differences, steering wheel angle and/or steering torque instead of from information from the device for determining the profile of the roadway. An interference from side wind or a laterally inclined roadway can be detected by a wheel speed difference which occurs when the steering angle is unchanged. A desired yaw rate which is necessary for compensating the deviation of the vehicle from a desired direction of travel, for example due to a gust of side wind, is determined from a model-based calculation or a characteristic diagram. The wheel contact forces are changed by actuating the actuating elements as a function of the desired yaw rate. Taking into account the steering torque permits the driver interventions to be taken into account and avoids overcompensation of interference.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.